This invention relates to urea flow control agents which are compatible with melamine cured urethane paint, and a method of making such agents. The invention further relates to high solids urethane paint compositions, the flow characteristics of which are modified by the incorporation of such flow control agents.
Earlier work by the inventors hereof led to the development of novel, ultraviolet (U.V.) stable urethane binder compositions which are the subject matter of U.S. Pat. No. 4,387,194, assigned to General Motors Corporation, issued June 7, 1983. One such composition that was used in the current invention is prepared by reacting a hindered piperidinol U.V. stabilizing agent with a substantial stoichiometric excess of alkyl polyfunctional isocyanate. This reaction yields a piperidine group terminated isocyanate prepolymer. The isocyanate prepolymer is then reacted with a stoichiometric excess of polyether polyol, preferably a mixture of polyoxypropylene diols and triols. The diol to triol ratio may be adjusted to provide desired hardness, toughness and flexibility in the cured paint. The colored paints are pigmented at a ratio of about 40 parts pigment per 100 parts resin.
The preferred cross-linking agent for the piperidinol U.V. stabilized paints is a partially methylolated melamine resin. In acid environments (pH preferably less than 5) and at elevated temperatures above about 250.degree. F. the melamine formaldehyde reacts rapidly with the hydroxy end groups of the U.V. stabilized urethane resin to form a complex, cross-linked polymeric structure.
These high solids paints are easy to apply to substrates by spray application even though their nonvolatile solids (resin and pigment) content may be well over 50%. However, their viscosities and thixotropic properties are such that after application, they are prone to run and sag when applied to vertically oriented substrates. Thus, improved flow control of these paint compositions is critical to their greater utility in automotive applications where coatings may be sprayed onto vertically oriented body panels or parts with complex surface contours.
To obtain acceptable flow control, the coating must have the proper rheology during the application, flash, and bake periods. The two major factors for proper flow control are film leveling and film sagging. Film leveling involves a coating flowing out after application so as to obliterate surface irregularities such as orange peel and cratering. Film sagging involves a downward movement due to gravity of a vertical paint film after application and before cure which results in an uneven coating having a thick lower edge. In an ideal situation, a coating will level completely but have no sag. However, those factors that give good leveling, such as low coating viscosity, also serve to promote sagging. A compromise between good leveling and good sag resistance is required.
Desired rheological behavior for a sprayed paint coating is that it has a very low viscosity when subjected to high shear in the paint spraying process. However, once the paint has been applied to a substrate, the viscosity should increase enough so that the paint does not sag or drip but not so much that it fails to level. With conventional low solids-high solvent enamel paint systems, rheological control is maintained by controlling the rate of solvent evaporation. However, such control is not possible with a high solids paint.
Furthermore, the thixotropic particulate and polymeric fillers used in conventional paint systems to control viscosity are not useful in our acid catalyzed, melamine cured urethane paints. When fillers were added in amounts adequate to effect thixotropy, it was only at the expense of other important coating properties such as gloss, durability, leveling, distinctness of image or spray solids concentration.
Accordingly, it is an object of this invention to provide a flow control agent which, when added to high solids, melamine cured urethane paints will improve the flow characteristics of such paints, particularly to promote ease of spraying and adequate leveling while inhibiting drip and sag.
It is another object of this invention to provide a method of making such flow control agents by reacting a mixture of polyoxypropylene diols and triols with an excess of methylene-bis-(4-cyclohexyl isocyanate) and thereafter reacting the product of this reaction with various substituted ethanolamines to create a urethane-urea polymer having strong hydrogen bonding and polar end groups.
It is yet another object of this invention to provide a high solids, melamine cured urethane paint formulation containing a urethane-urea flow control agent such that the paint is sprayable in a production environment, curable in a relatively short period of time at relatively low temperatures, and spreadable when wet without dripping or sagging.